1. Field of the Invention
This invention relates to a process for effecting mass transfer between a liquid phase and a gaseous phase in a filled-type column wherethrough said phases are caused to flow in a countercurrent relationship.
In the description given below and in the following claims, the term: “filled-type column”, is understood to mean—in general—an apparatus having, disposed on its interior, a plurality of variously shaped and sized elements (filler) at whose surfaces a liquid phase and a gaseous phase are caused to contact each other in order to effect mass transfer; apparatus of this type are widely employed in chemical plants, e.g. for decomposing, absorbing, distilling, and scrubbing chemicals.
The invention also relates to a filled-type column for implementing the above process, and to a method of retrofitting a pre-existing column for conversion into a filled-type column according to the invention.
In the description given below and in the following claims, the term: “retrofitting”, is understood to mean the in-situ modification of a pre-existing column of the filled or plates type to improve its performance and, for example, to increase its capacity and/or its efficiency in mass transfer between the liquid phase and the gas phase, as well as to reduce its energy consumption.
As is known, in the field there is increasingly felt the need of providing processes easy to implement, which can effect mass transfer between a liquid phase and a gaseous phase in a simple and efficient way, at low investment and operating costs and with a low energy consumption.
2. Prior Art
In order to meet the above requirement, processes for effecting mass transfer have been proposed in the field wherein a liquid phase and a gaseous phase are caused to flow through a filled-type column in a downward and an upward (substantially axial) direction, respectively.
While being simple to implement, such prior processes are beset with problems arising mainly from the large pressure drop experienced by the gas phase in going through the filler.
The filler is, in fact, disposed inside substantially cylindrical columns having a large height-to-diameter ratio intended to ensure an interphase contact of sufficient duration for the phases to enhance the mass transfer.
Consequently, in flowing through the filler, the gas phase undergoes a significant reduction in pressure (pressure drop), which places limitations on the amount of gas that can be fed into the column, thus curtailing the column capacity.
By reason of this pressure drop, the flow rate of the gas phase fed into the column must be held below predetermined values, the overtaking of which would result in the appearance of an undesirable “flooding” phenomenon, whereby the column becomes flooded with liquid phase which is held back in its downward movement by frictional drag from the gas phase. It will be appreciated that in this condition the column would no longer be operable, due to the mass transfer dropping to virtually nil.
In other words, the large pressure drop of the gaseous phase in going through the filler is a limiting factor of the gas phase flow rate through the column designed to implement the above prior art processes, and hinders an effective mass transfer between the phases.
To counteract the pressure drop in the gaseous phase, and thus to increase the capacity of filled-type columns according to the prior art, some special fillers having a high void ratio have been proposed, wherethrough the gaseous phase stream undergoes no large pressure drops.
While such fillers allow the flow rate of the gas phase being fed into the column to be increased somewhat, they have been unsuccessful in providing a fully satisfactory mass transfer between the phases, because the pressure drop of the gas phase through the column is still quite significant, as are the operational constraints due to the aforementioned flooding phenomenon.
Columns of the so-called plates type, i.e. provided with a plurality of perforated plates fitted horizontally inside the column, have been proposed alternatively to the filled-type columns.
In this case, the process for effecting mass transfer between the liquid and gaseous phases comprises mixing the phases together in the plates, the plates being typically designed to enhance the mutual contact of the phases.
Plates-type columns have been found suitable for low liquid flow-rate applications, but are liable to become flooded, disallowing any further effective mass transfer between the liquid and gas phases.
It is on account of the above disadvantages that prior art processes for effecting mass transfer between a liquid phase and a gaseous phase have provided unsatisfactory performance heretofore, both in terms of overall efficiency of the transfer between the phases, energy consumption, and operating and investment costs of the columns used to implement such processes (which are, as said before, beset with several limitations).
All this, despite this technology having been utilized in a variety of chemical applications for years and the aforementioned need being increasingly felt in the field.